Reversible phosphorylation of proteins is a prevalent biological mechanism for modulation of enzymatic activity in living organisms. Tonks et al., J. Biol. Chem., 263(14):6722-30 (1988). Such reversible phosphorylation requires both a protein kinase (PK), to phosphorylate a protein at a particular amino acid residue, and a protein phosphatase (PP), to remove the phosphate moieties. See generally, Hunter, Cell, 80:225-236 (1995). Recently, it has been estimated that humans have as many as 2000 conventional PK genes, and as many as 1000 PP genes. Id.
One major class of PK's/PP's--the protein serine/threonine kinases and protein serine/threonine phosphatases--have been shown to play critical roles in the regulation of metabolism. See generally, Cohen, Trends Biochem. Sci., 17:408-413 (1992); Shenolikar, Ann. Rev. Cell Biol., 10:55-86 (1994); Bollen et al., Crit. Rev. Biochem. Mol. Biol., 27:227-81 (1992). As their name suggests, these enzymes phosphorylate and dephoshphorylate serine or threonine residues of substrate proteins. Inhibitors of protein serine/threonine phosphatases and kinases have been described. See, e.g., MacKintosh and MacKintosh, TIBS, 19:444-448 (1994).
The protein tyrosine kinases/phosphatases comprise a second, distinct family of PK/PP enzymes of significant interest, and have been implicated in the control of normal and neoplastic cell growth and proliferation. See Fisher et al., Science, 253:401-406 (1991). Protein tyrosine kinase (PTK) genes are ancient in evolutionary origin and share a high degree of inter-species conservation. See generally Hunter and Cooper, Ann. Rev. Biochem., 54:897-930 (1985). PTK enzymes exhibit high specificity for tyrosine, and ordinarily do not phosphorylate serine, threonine, or hydroxyproline.
More than 75 members of the PTPase family have been identified in eukaryotes, prokaryotes, and even viruses. Tonks and Neel, Cell 87:365-368. Protein tyrosine phosphatases (PTPases) were originally identified and purified from cell and tissue lysates using a variety of artificial substrates, and therefore their natural functions and substrates were not obvious. However, their roles in cellular processes, including cell-cell contact and cell adhesion, and growth factor and antigen signaling events, have begun to be elucidated.
PTPases are generally grouped into two categories: those which have both an extracellular domain and an intracellular catalytic domain, the receptor PTPases (R-PTPases); and those which are entirely intracellular. For R-PTPases much effort has been directed at determining the function of the extracellular domain. Most of the R-PTPases contain extracellular domains which are structurally similar to domains found in known adhesion molecules; these domains include fibronectin type III repeats, immunoglobulin domains, and cadherin extracellular repeats. See generally Brady-Kalnay and Tonks, Curr. Opin. Cell. Biol. 7:650-657 (1995); Streuli, Curr. Opin. Cell. Biol. 8:182-188 (1996). This homology with proteins known to be involved in adhesion suggested a role for these R-PTPases in regulating or mediating adhesion events. For several of the R-PTPases, this has now been demonstrated.
Cells form specialized structures at the sites of cell-cell contact (adherens junctions) and cell-extracellular matrix contact (focal adhesion). Multiple signal transduction molecules are recruited to these sites, including several PTK's; and these sites are characterized by increased protein tyrosine phosphorylation. These sites are impermanent, and are created and destroyed as required for cell mobility. As enhanced tyrosine phosphorylation is characteristic of the formation of adherens junctions and focal adhesions, it is likely that protein tyrosine dephosphorylation by PTPases serves to regulate the creation and destruction of the sites. Supporting this, several studies have shown that treatment with a general PTPase inhibitor (vanadate) resulted in increased focal adhesion formation and increased cell spreading. Volberg et al., The EMBO J. 11:1733-1742 (1992); Bennett et al., J. Cell Sci. 106:891-901 (1993). Importantly, the broadly-expressed LAR R-PTPase has been demonstrated to localize to focal adhesions, apparently via the LAR-interacting protein LIP.1. Serra-Pages et al., The EMBO J. 14:2827-2838 (1995). As PTP.delta. and PTP.sigma., both R-PTPases, also associate with LIP.1 [Pulido et al., Proc. Natl. Acad. Sci. 92:11686-11690 (1995)], it is likely that these two phosphatases can also localize to focal adhesions. Most significantly, LAR only localized to the portion of the focal adhesion which is proximal to the nucleus, and is thought to be undergoing disassembly. Thus it is likely that these phosphatases act to negatively regulate focal adhesion formation, acting to enhance the destruction of the focal adhesion site.
R-PTPases may also act to positively regulate adhesion. Adherens junctions contain, among others, adhesion receptors termed cadherins which mediate cell-cell contact through homophilic binding; the cadherins associate with .alpha.-, .beta.-, and .gamma.- catenins, intracellular proteins which interact with cortical actin. Association between cadherins and catenins serves to stabilize the adherens junction and to strengthen cell-cell contact. See generally Cowin, Proc. Natl. Acad. Sci. 91:10759-10761 (1994). Association of cadherin with .beta.-catenin is decreased by tyrosine phosphorylation of .beta.-catenin [Kinch et al., J. Cell. Biol. 130:461-471 (1995); Behrens et al., J. Cell. Biol. 120:757-766 (1993)]; moreover, treatment with the PTPase inhibitor vanadate inhibits cadherin-dependent adhesion [Matsuyoshi et al., J. Cell. Biol. 118:703-714 (1992)]. Collectively, these data indicate that PTPase activity is critical in maintaining cadherin-mediated cell aggregation. The R-PTPases PTP.mu. and PTP.kappa. associate intracellularly with cadherins, and colocalize with cadherins and catenins to adherens junctions [Brady-Kalnay et al., J. Cell. Biol. 130:977-986 (1995); Fuchs et al., J. Biol. Chem. 271:16712-16719 (1996)]; thus PTP.mu. and PTP.kappa. are likely to enhance cadherin function by limiting catenin phosphorylation.
In addition to their catalytic function in regulating adhesion events, several R-PTPases have direct roles in mediating adhesion through their extracellular domains. PTP.kappa. and PTP.mu. mediate cellular aggregation through homophilic binding [Brady-Kalnay et al., J. Cell. Biol. 122:961-972 (1993); Gebbink et al., J. Biol. Chem. 268:16101-16104 (1993); Sap et al., Mol. Cell. Biol. 14:1-9 (1994)]. The neuronal PTP.zeta. (which has also been called R-PTP.beta.) binds to contactin, a neuronal cell recognition molecule; binding of PTP.zeta. to contactin increases cell adhesion and neurite outgrowth. Peles et al., Cell 82:251-260 (1995). A secreted splice variant of PTP.zeta. (also known as phosphacan) binds the extracellular matrix protein tenascin [Barnea et al. J. Biol. Chem. 269:14349-14352 (1994)], and the neural cell adhesion molecules N-CAM and Ng-CAM [Maurel et al., Proc. Natl. Acad. Sci. 91:2512-2516 (1994)]. As the expression of PTP.zeta. is restricted to radial glial cells in the developing central nervous system, which are though to form barriers to neuronal migration during embryogenesis, it is likely that the interaction of PTP.zeta. with contactin, tenascin, N-CAM, and/or Ng-CAM acts to regulate neuronal migration. This has been demonstrated for a related R-PTPase, DLAR, in Drosophila [Krueger et al. Cell 84:611-622 (1996)].
Because tyrosine phosphorylation by PTK enzymes usually is associated with cell proliferation, cell transformation and cell differentiation, it was assumed that PTPases were also associated with these events. For several of the intracellular PTPases, this function has now been verified.
SHP1 (which has also been called SHPTP1, SHP, HCP, and PTP-1C [see Adachi et al., Cell 85:15 (1996)]), an intracellular PTPase which contains two amino-terminal phosphotyrosyl binding Src Homology 2 (SH2) domains followed by the catalytic PTPase domain, has been demonstrated to be an important negative regulator of growth factor signaling events. See generally Tonks and Neel, supra, Streuli, supra. In mice, loss of SHP1 function (the motheaten and viable motheaten phenotypes) causes multiple hematopoietic defects resulting in immunodeficiency and severe autoimmunity; culminating in lethality by 2-3 weeks or 2-3 months depending on the severity of SHP1 deficiency. Although these mice have reduced numbers of hematopoietic cells, suggesting defects in development and maturation, those cells which survive and enter the periphery are characterized by hyper-responsiveness to growth factors and antigen. This observation suggested a role for SHP1 in negative regulation of hematopoietic signaling events.
This has now been well-established for the erythropoietin receptor (EpoR), a member of the cytokine receptor family (which also includes the receptors for interleukins 2, 3, 4, 5, 6, 7; granulocyte-macrophage colony stimulating factor, and macrophage colony stimulating factor). SHP1 associates via its SH2 domains with tyrosine-phosphorylated EpoR, causing dephosphorylation and inactivation of the EpoR-associated Janus kinase 2 and termination of the cellular response to erythropoietin. Klingmuller et al., Cell 80:729-738 (1995). Mutation of the tyrosine on the EpoR to which SHP1 binds results in enhanced cell proliferation to erythropoietin in vitro [Klingmuller, supra]. In humans, mutation of the EpoR resulting in loss of association with SHP1 causes autosomal dominant benign erythrocytosis, which is characterized by increased numbers of erythrocytes in the periphery and increased hematocrit. de la Chapelle et al., Proc. Natl. Acad. Sci. 90:4495-4499 (1993).
SHP1 also appears to be a negative regulator of the cellular response to colony stimulating factor-1 (CSF-1, a major macrophage mitogenic cytokine), as cells from viable motheaten and motheaten mice, which have reduced or absent SHP1 function, are hyper-responsive to CSF-1 in vitro. Reduced SHP1 expression also results in increased cellular response to interleukin 3 [Yi et al., Mol. Cell. Biol. 13:7577-7586 (1993)]. Collectively, these observations suggest that SHP1 functions to limit the cellular response to cytokines and growth factors by reversing the tyrosine phosphorylation of key signaling intermediates in these pathways.
PTPases appear to play a homologous role in the insulin signaling pathway. Treatment of adipocytes with the PTPase inhibitor vanadate results in increased tyrosine phosphorylation and tyrosine kinase activity of the insulin receptor (InsR), and enhances or mimics the cellular effects of insulin including increased glucose transport. See, e.g., Shisheva and Shechter, Endocrinology 133:1562-1568 (1993); Fantus, et al., Biochemistry 28:8864-8871 (1989); Kadota, et al., Biochem. Biophys. Res. Comm. 147:259-266 (1987); Kadota, et al., J. Biol. Chem. 262:8252-8256 (1987). Transiently induced reduction in expression of two PTPases, the intracellular PTPase PTP-1B and the R-PTPase LAR, resulted in similar increases in the cellular response to insulin. Kulas, et al., J. Biol. Chem. 270:2435-2438 (1995); Ahmad et al., J. Biol. Chem. 270:20503-20508 (1995). Conversely, increased cellular expression of several PTPases (PTP.alpha., PTP.epsilon., CD45) in vitro has been demonstrated to result in diminished InsR signaling [see, e.g., Moller, et al., J. Biol. Chem. 271:23126-23131 (1995); Kulas et al., J. Biol. Chem. 271:755-760 (1996)]. Finally, increased expression of LAR was observed in adipose tissue from obese human subjects [Ahmad, et al., J. Clin. Invest. 95:2806-2812 (1995)]. These data provide clear evidence that PTPases negatively regulate the insulin signaling pathway.
While many of the PTPases function to negatively regulate cellular metabolism and response, it is becoming increasingly evident that PTPases provide important positive signaling mechanisms as well. Perhaps the best example of such a positive regulator is the hematopoietic R-PTPase CD45. See generally Streuli, supra; Okumura and Thomas, supra; Trowbridge, Annu. Rev. Immunol. 12:85-116 (1994). CD45 is abundantly expressed on the cell surface of all nucleated hematopoietic cells, in several alternative splice variants. T and B lymphocytes which lack CD45 expression are incapable of responding normally to antigen, suggesting that CD45 is required for antigen receptor signaling. Genetically engineered mice which lack expression of CD45 exhibit severe defects in T lymphocyte development and maturation, indicating an additional role for CD45 in thymopoiesis. The major substrates for CD45 appear to be members of the Src family of PTK's, particularly Lck and Fyn, whose kinase activity is both positively and negatively regulated by tyrosine phosphorylation. Lck and Fyn isolated from CD45-deficient cells are hyperphosphorylated on negative regulatory tyrosine residues, and their PTK activity is reduced. As CD45 can dephosphorylate and activate purified Lck and Fyn in vitro, these data suggest that CD45 maintains the activity of Lck and Fyn in vivo through dephosphorylation of these negative regulatory tyrosines and that this is an important mechanism for maintaining lymphocyte homeostasis.
A second PTPase which is now believed to play an important positive role in signal transduction is the intracellular, SH2-domain-containing SHP2 (which has also been called SHPTP-2, SHPTP-3, syp, PTP2c, and PTP-1D [Adachi, et al., supra]). See generally Saltiel, Am. J. Physiol. 270:E375-385 (1996); Draznin, Endocrinology 137:2647-2648. SHP2 associates, via its SH2 domains, with the receptor for platelet-derived growth factor (PDGF-R), the receptor for epidermal growth factor (EGF-R), with the insulin receptor, and with the predominant substrate of the InsR, insulin receptor substrate 1 (IRS1). Bennett, et al., Proc. Natl. Acad. Sci. 91:7335-7339 (1994); Case, et al., J. Biol. Chem. 269:10467-10474 (1994); Kharitonenkov, et al., J. Biol. Chem. 270:29189-29193 (1995); Kuhne, et al., J. Biol. Chem. 268:11479-11481 (1993). SHP2 PTPase activity is required for cellular response to EGF and insulin, as competitive expression of inactive forms of SHP2 results in diminished signaling events and reduced cellular responses to EGF and insulin. Milarski and Saltiel, J. Biol. Chem. 269:21239-21243 (1994); Xiao et al., J. Biol. Chem. 269:21244-21248 (1994); Yamauchi et al., Proc. Natl. Acad. Sci. 92:664-668 (1995). The relevant substrate(s) for the PTPase domain of SHP2 is not known.
Due to the fundamental role that PTPases play in normal and neoplastic cellular growth and proliferation, a need exists in the art for agents capable of modulating PTPase activity. On a fundamental level, such agents are useful for elucidating the precise role of protein tyrosine phosphatases and kinases in cellular signalling pathways and cellular growth and proliferation. See generally MacKintosh and MacKintosh, TIBS, 19:444-448 (1994).
More importantly, modulation of PTPase activity has important clinical significance. For example, PTP-1B overexpression has been correlated with breast and ovarian cancers [Weiner et al., J. Natl. Cancer Inst., 86:372-8 (1994); Weiner et al., Am J. Obstet. Gynecol., 170:1177-883 (1994)], and thus agents which modulate PTP-1B activity would be helpful in elucidating the role of PTP-1B in these conditions and for the development of effective therapeutics against these disease states. The important role of CD45 in hematopoietic development and T lymphocytc function likewise indicates a therapeutic utility for PTPase inhibitors in conditions that are associated with autoimmune disease, and as a prophylaxis for transplant rejection. The antibiotic suramin, which also appears to possess anti-neoplastic indications, has recently been shown to be a potent, irreversible, non-competitive inhibitor of CD45. See Ghosh and Miller, Biochem. Biophys. Res. Comm. 194:36-44 (1993). The negative regulatory effects of several PTPases on signaling through receptors for growth factors and cytokines, which are implicated in normal cell processing as well as disease states such as cancer and atherosclerosis, also indicate a therapeutic potential for PTPase inhibitors in diseases of hematopoietic origin.
The PTPase Yop2b is an essential virulence determinant in the pathogenic bacterium Yersinia, responsible for bubonic plague. Bliska et al., Proc. Natl. Acad Sci. USA, 88:1187-91 (1991), and thus an antimicrobial indication exists for PTPase inhibitor compounds, as well.
PTPases have been implicated in diabetic conditions. Experiments with one family of PTPase inhibitors, vanadium derivatives, indicate a therapeutic utility for such compounds as oral adjuvants or as alternatives to insulin for the treatment of hyperglycemia. See Posner et al., J. Biol. Chem., 269:4596-4604 (1994). However, such metal-containing PTPase inhibitors act in a fairly non-specific fashion and act with similar potencies against all PTPase enzymes.
In addition to vanadium derivatives, certain organic phosphotyrosine mimetics are reportedly capable of competitively inhibiting PTPase molecules when such mimetics are incorporated into polypeptide artificial PTPase substrates of 6-11 amino acid residues. For example, a "natural" (phosphorylated tyrosine) PTPase substrate, which may be depicted by the Formula: ##STR2## has been mimicked by eleven-mer oligopeptides containing phosphonomethyl phenylalanine (Pmp), as depicted by the schematic Formula: ##STR3## See Chatterjee et al., "Phosphopeptide substrates and phosphonopeptide inhibitors of protein tyrosine phosphatases," in Peptides: Chemistry and Biology (Rivier and Smith, Eds.), 1992, Escom Science Publishers: Leiden, Netherlands, pp. 553-55; Burke et al., Biochemistry, 33:6490-94 (1994). More recently, Burke et al., Biochem. Biophys. Res. Comm. 204(1):129-134 (1994) reported that a particular hexameric peptide sequence comprising a Pmp moiety or, more preferably, a phosphonodifluoromethyl phenylalanine (F.sub.2 Pmp) moiety, as depicted by the schematic Formula: ##STR4## competitively inhibited PTP-1B. However, such hexapeptide inhibitors nonetheless possess drawbacks for PTPase modulation in vivo. More particularly, the hexapeptide inhibitors described by Burke et al. are sufficiently large and anionic to potentially inhibit efficient migration across cell membranes, for interaction with the catalytic domains of transmembrane and intracellular PTPase enzymes which lie within a cell membrane. A need exists for small, organic-molecule based PTPase inhibitors having fewer anionic moieties, to facilitate migration across cell membranes.
For all of the foregoing reasons, a need exists in the art for novel compounds effective for modulating, and especially inhibiting, the phosphatase activity of protein tyrosine phosphatase molecules.